The present invention relates to a method and system for safely shutting down and starting up an optical communication line that includes optical fiber amplifiers. In particular, the present invention relates to a technique for ensuring that light signals emanating from fiber amplifiers in an optical communication line remain at a safe level during system maintenance or shut down.
Optical communication systems conventionally involve the transmission of light signals across a distance of optical fiber between a transmitter terminal and a receiver terminal. For transmission across at least several tens of kilometers, amplification of the light signals occurs to offset attenuation within transmission optical fibers and intermediary equipment. One technique for amplifying light signals uses opto-electronic regenerators within the optical path to convert the light to an electrical signal, amplify and re-shape the electrical signal, and then convert the electrical signal back to optical form.
Modern optical systems employ amplifiers of optical fibers doped with rare-earth elements, such as erbium. These fiber amplifiers generate a significant amount of optical power within an optical fiber under normal operating conditions. Fiber amplifiers in general operate by exciting ions of the rare-earth dopant with a characteristic pump wavelength so that the excited ions transfer energy to optical signals passing through the amplifier at a different characteristic wavelength. For erbium-doped fiber amplifiers, a characteristic pump wavelength is about 980 nm or 1480 nm, and a characteristic transmission wavelength is about 1550 nm.
If a disruption in a fiber line occurs, however, the fiber amplifiers can cause high levels of optical power, which can be harmful to the human eye, to emanate from the fiber into the surrounding environment. Such a disruption can arise from an accidental break in a section of the transmission line or by a purposeful disconnection of the line during system maintenance or repair. In either event, care must be taken to protect against directing emission of light from the fiber into a person's eyes.
Several documents have considered how to avoid the emission of high levels of optical power from the end of a disrupted fiber. U.S. Pat. No. 5,278,686 and U.S. Pat. No. 5,355,250, for example, disclose techniques for shutting down an entire optical communication line in the event of a fiber break. The shutdown can occur by disabling the source of optical signals at the transmitter or by disabling the pump source for the fiber amplifiers along the line.
In the '686 patent, a bidirectional optical fiber system has two terminals that each have a transmitter and a receiver for the respective opposite transmission directions in the system. The transmitter and receiver in each terminal are interconnected by a protective device that deactivates the transmitter when the associated receiver does not receive an optical signal. Each amplifier in the system also includes a device that detects the presence of passing light signals at the amplifier and interrupts the line when the light energy drops below a predetermined level. A fiber break will cause each amplifier in a transmission chain to shut down sequentially until the entire system is disabled. Conversely, repair of the fiber will cause the cascaded amplifiers to start up sequentially as they detect the rise in signal power. Similarly, the '250 patent discloses a device that detects a loss in optical power upstream of a fiber amplifier and then reduces or eliminates the emission from the amplifier by shutting down the pump source for the amplifier:
U.S. Pat. No. 5,428,471 and ITU Recommendation G.958 disclose a technique of interlocking amplifiers surrounding a fiber break. The interlock occurs by communicating failure information between amplifiers at a common site that serve lines traveling in opposite directions. As shown in FIG. 1 of the '471 patent, a disruption to fiber 14 at location “A” is detected by amplifier 40b due to a drop in optical input power, which causes amplifier 42a within fiber 16 to shut down or reduce its output power to a safe level. Shut down of amplifier 42a is similar to a cable disruption at location “8” in cable 16 and is detected by amplifier 42b, which causes amplifier 40a to shut down. As a result, optical energy at location “A” is terminated. The '471 patent explains that for the interlock of amplifiers in a two-way system, only two amplifiers (42a and 40a ) are disabled, and amplifiers not located adjacent to the fiber disruption remain operative.
The '471 patent further discloses a continuity signal of a safe power level generated on fibers 14 and 16 at the output of each of the amplifiers that is used to sense fiber disruption. The amplifiers also use the continuity signal to sense repair of the break to actuate communication along the previously disrupted line.
WO 98/25361 discloses a fiber amplifier with a pump unit that provides a nominal, continuous pump power in an operational state but changes its mean pump power in a safety state to give a pulsed signal with a power below a prescribed safety limit. Upon startup, the pump unit in WO 98/25361 will first assume a safety state to verify that re-connection to the fiber amplifier has occurred, then the unit will escalate to an intermediate power state until the system receiver obtains a transmission signal. Thereafter, the pump unit raises its power to the full operational condition.
Applicants have observed that shutting down individual amplifiers in an optical transmission system in cascade as suggested in the '686 patent and the '250 patent may take too long to isolate a fault. Under current international standards such as IEC 60825-2, when an optical cut occurs an optical safety circuit with a reasonable reliability has to reduce the optical power to a predetermined level in the direction toward the fiber cut within 1 second.
Also, Applicants have observed that an amplifier interlock system as suggested in the '471 patent and ITU Recommendation G.958, which deactivates only one amplifier in each transmission direction, allows amplified spontaneous emission (ASE) noise to accumulate within the remaining activated amplifiers. Subsequent insertion of an optical signal to the activated amplifiers can result in optical pulses of very high peak power, which may endanger connectors or photodiodes within the line.
Some publications have recognized a condition of large pulses with optical amplifiers. For example, an article by Tokura et al. entitled “Quantitative Analysis of Optical Surge Propagation on Transmission Systems,” IOOC-ECOC '97, Vol. 3, pp. 263-66 (1997) investigates optical surges in fiber amplifiers having a large gain. This article, however, concludes that a maximum value of optical surge can be suppressed by regulating the switching time constants of an input signal and concludes that a system is safe from optical surge propagation when a switching speed of 10 μsec or slower is used for a signal input.
Also, U.S. Pat. No. 5,317,660 discloses an optical transmission system with an optical filter for providing protection against giant pulses that may develop from a fiber break. As explained in this patent, though, the giant pulses are caused by reflection of spontaneous emission at the point of the fiber break, and the use of optical isolators avoids the reflection of light back through the amplifier and the subsequent emission of giant pulses.